In Japan, only 30% of the primary energy supply is used as effective energy, with about 70% being eventually lost to the atmosphere as heat. The heat generated by combustion in industrial plants, garbage-incineration facilities or the like is lost to the atmosphere without conversion into other energy. In this way, a vast amount of thermal energy is wastefully discarded, while acquiring only a small amount of energy by combustion of fossil fuels or other means.
To increase the proportion of energy to be utilized, the thermal energy currently lost to the atmosphere should be effectively used. For this purpose, thermoelectric conversion, which directly converts thermal energy to electrical energy, is an effective means. Such a thermoelectric conversion utilizes the Seebeck effect, and is an energy conversion method for generating electricity in which a difference in electric potential is caused by creating a difference in the temperature between both ends of a thermoelectric material.
In such a method for generating electricity utilizing thermoelectric conversion, i.e., thermoelectric generation, electricity is generated simply by setting one end of a thermoelectric material at a location heated to a high temperature by waste heat, and the other end in the atmosphere (room temperature) and connecting conductive wires to both ends. This method entirely eliminates the need for moving parts such as the motors or turbines generally required for electric power generation. As a consequence, the method is economical and can be carried out without generating gases by combustion. Moreover, the method can continuously generate electricity until the thermoelectric material has deteriorated. Furthermore, thermoelectric generation enables electric power generation at a high power density. Therefore, it is possible to make electric power generators (modules) small and light enough to use them as mobile power supplies for cellular phones, notebook computers, etc.
Therefore, thermoelectric generation is expected to play a role in the resolution of future energy problems. To realize thermoelectric generation, a thermoelectric module comprising thermoelectric materials that have both a high thermoelectric conversion efficiency and excellent properties in terms of heat resistance, chemical durability, etc., will be required.
CoO2-based layered oxides such as Ca3Co4O9 have been reported as substances that achieve excellent thermoelectric performance in air at high temperatures, and such thermoelectric materials are currently being developed (see R. Funahashi et al., Jpn. J. Appl. Phys., 39, L1127 (2000), for example).
The realization of efficient thermoelectric generation using such thermoelectric materials requires a thermoelectric element comprising a pair of connected p- and n-type thermoelectric materials, and a thermoelectric module obtained by integrating thermoelectric elements, i.e., an electric power generator. However, the development of thermoelectric elements and thermoelectric modules has been delayed so far as compared to the development of thermoelectric materials.
In particular, the development of a method for connecting thermoelectric materials with a low electrical resistance is important for putting thermoelectric modules into practical use. In the case of thermoelectric generation using high-temperature waste heat of 673 K (400° C.) or higher, thermoelectric materials are connected using, as a binder, a paste containing a noble metal such as silver, gold, or platinum because a connecting portion formed by soldering is likely to oxidize or melt under such conditions. However, such noble metal pastes are not suitable when oxides are used as substrate materials, thermoelectric material, etc. because there is a large difference in the thermal expansion coefficient between the oxide and the noble metal contained in the paste. Thus, repeated high-temperature power generations cause separation at the connecting portion, resulting in increased internal resistance and lowered mechanical strength. The connecting portion therebetween also has a problem of a large interface resistance due to contact between the metal and oxide.